Of the steels I've used (ranked best to worst, top to bottom):
S30V (on a MT Scarab) is the best. The knife broke before I ever needed to sharpen it. When I get it back I hope to use it much more.
154CM (on a Woodard Fer De Lance). Before I sold the knife (to fund the Scarab), I was extremely impressed by the steel on this knife. Took a scary-sharp hair popping edge without much effort, and held it for a long time.
Victorinox SAK steel (whatever the hell it is). Not the best at holding an edge, but could get sharper than any other steel I've used.
440C on BM balisongs. Good steel, but takes more maintainance than I care for. Seems like it might be microscopically chipping or something. Honestly, I think it would be better if it were a bit softer (57-58 Rc instead of 59-60). The steel on my 31S seems better than that on my 42.
440A (on Kershaw Leek). Could get scary sharp, but held edge poorly.
440C on S&W knife. Never mind that the rest of the knife was garbage, the steel was too. Except for using a extra-coarse diamond, I couldn't get this knife sharp worth a rat's ass. And then, it was more like a saw blade than a knife. Utter junk.
AUS-6 on various Kershaws and CRKTs. Got nice and sharp, but had the edge holding ability of a stick of butter (in Arizona, in the summer). I could profile at a 45 degree and it'd still get a wire edge.
I've sharpened VG-10 (Spyderco), which took a razor edge easily, but I've never used it for any real period of time. I hope to in the near future though.